This invention relates to a method of producing a semiconductor device such as MOSIC (Metal-Oxide-Semiconductor Integrated Circuit), and more particularly to a technique for forming a substrate electrode by simple treatments. In this specification, the term "MOS" shall be used in a broad sense including the three-layered structure of semiconductor-insulator-conductor.
The substrate electrode of a MOSIC device is usually disposed on the lower surface of a substrate, that is, on a surface opposite to a surface (upper surface) in which source and drain regions or an interconnection layer are/is formed.
On the other hand, in a MOSIC device requiring a high reliability, a resin mold package is often replaced with a ceramic package or a glass package as a sealing member for the device.
In this regard, there has been a problem in the case where the MOSIC device having the substrate electrode on the rear surface of the substrate is sealed with such ceramic package or glass package.
More specifically, in the case of employing the above package, an external lead and the substrate electrode must be connected by means of a wire. However, the wire bonding is very difficult because of the presence of the substrate electrode on the rear substrate surface. For this reason, the speed-up of the automatic wire bonding cannot be achieved satisfactorily.
It has therefore been considered to dispose the substrate electrode on the front surface of the substrate in the case of sealing the device with the ceramic package or the glass package.
Especially, in a silicon gate MOSIC device having a LOCOS (Local Oxidation of Silicon) structure, a source or drain region having a conductivity type opposite to that of a substrate is formed in the front surface of the substrate other than a field oxide film as well as a silicon gate electrode, and hence, there has been considered a method wherein a part of the field oxide film is etched to expose the front substrate surface and wherein the substrate electrode is formed on the exposed front substrate surface.
This method of forming the substrate electrode, however, has had such disadvantages that the stage of work for etching and removing the thick field oxide film needs to be specially added and that the etching and removal of the field oxide film is not easy and takes a long time.
The inventor of this invention took note of the fact that any semiconductor region having the conductivity type opposite to that of the substrate is not formed underneath the gate electrode. This invention has been achieved as the result.